1. Field of the Invention
The present invention relates to an electret capacitor microphone.
2. Prior Art
Electret capacitor microphones are often provided with a thin diaphragm having a thin metal film facing toward an opening in a metal casing and a fixed electrode opposed thereto, and utilize the principal of a change in capacity between the diaphragm and the fixed electrode dependently on the vibration of the diaphragm due to a sound wave. The diaphragm utilizes a thin metallized electret material on a surface thereof, and a capacitor is formed between the diaphragm and the fixed electrode. A change in capacity is converted into a current or voltage change through a solid state device, the sound wave being fetched as an electric signal.
Conventional electret capacitor microphones have been provided in a cylindrical metal casing 5 which disposes a diaphragm 4 made of an electret material with a metal layer deposited or other method on the surface and has a sound hole 21 in the central part of an upper surface facing to the diaphragm 4, as shown in FIG. 13. In the casing, a fixed electrode 6 is fixed just behind the diaphragm 4 with a capacitance gap 32 through a thin gap ring spacer 61 between the fixed electrode 6 and the diaphragm 4. Furthermore, a depressed dielectric 27 accommodating a solid state device 8 and a printed circuit board 29 are located in a lower part of the casing 5, and the casing is crimped to a peripheral edge 52 on the back face of the printed circuit board 29, using a crimping portion 52 of the opening of the metal casing 5.
In the conventional example, an input lead 81 of the solid state circuit device 8 is connected to a fixed electrode 26, and an output lead 82 penetrates through the special dielectric 27 and the printed circuit board 29 and is connected to a pad electrode 83 on a bottom face. On the other hand, the diaphragm 4 is conducted to the metal casing 5 through a film ring 41.
Recently, electret capacitor microphones have been required to be light and small-sized. However, in the conventional microphone technique, a large number of parts have been required and the thickness the crimping portion and the pad electrode could not be reduced.
Moreover, in the conventional microphones, since the dielectric and the printed circuit board have been utilized, it has been impossible to satisfy the demand for a reduction in size and weight of the whole microphone.
Conventional electret capacitor microphones are not suitable for mass-production and have had a low productivity. The reason is because parts from the fixed electrode 6 to the printed circuit board 29 are previously assembled as an amplifier block and each part of the assembly is individually disposed in the metal casing 5 and is finally crimped with the crimped portion 52.
In the conventional electret capacitor microphones, since the metal casing is crimped with a substrate, and then the uncertainty of fixation of each part causes microphone failure in use and reduces the yield of the microphone products.
Conventionally, soldering pad electrodes 83, which are takeoff connection portions, have been attached neat a center of the circuit board 29 so that it has been hard to carry out mounting a solid circuit device 8 on the circuit board 29 by soldering the pad electrode 83, and the conventional microphones have had a risk of short-circuiting the circuit board.
A main component of the microphone restricting a reduction in size has been a metal casing 5. A solid circuit device 8 including a transistor or an integrated circuit which normally has a square shape is accommodated on the circuit board with useless space because a conventional metal casing is cylindrical. In addition, the cylindrical metal casing 5 is hard to handle for mounting and to position. Furthermore, even if a sound hole is to have a complicated shape, processing is hard.
The metal casing 5 is apt to transmit a surrounding mechanical vibration, has an adverse influence on the diaphragm and supplies a noise to an electric signal, resulting in a deterioration in microphone performance.
Since the output leads was provided through the printed circuit board, pad electrodes for soldering was required to bury holes through which the leads pass airtightly.
A diaphragm 24 is a membrane of fluoro-polymer membrane such as polyfluoroethylene, polypropylene (FEP), or polyester polymer such as Mylar (registered trademark), and has a thermal resistance limited. Therefore, it has been hard to solder the microphone onto the printed circuit board by heating.
An object of the present invention is to provide a compact and thin electret capacitor microphone by reducing the number of parts constituting the capacitor microphone as much as possible.
Another object of the present invention is to provide an electret capacitor microphone capable of being assembled with mechanical fixation and electrical connection of parts carried out readily.
Another object of the present invention is to provide an electret capacitor microphone capable of being simply and reliably mounted in a small size onto another printed circuit board.
The elecret capacitor microphone of the present invention includes a casing formed of a dielectric and having a bottomed hollow portion which accommodates a combination of a diaphragm and a fixed electrode apart from each other at regular capacitance interval and a solid state device fox converting, into an electric signal, a change in an electrostatic capacity between a diaphragm and the fixed electrode, thereby forming an electret capacitor microphone.
Electret capacitor microphone will be hereinafter referred to as microphone, simply.
A dielectric casing can directly fix the parts in the hollow portion, eliminating the need for use of a printed circuit board for fixing a special insulator member and solid state circuit according to the conventional example, thereby decreasing the number of parts to be used.
Furthermore, an opening of the casing can be covered with a metal cover having a sound hole to penetrate sound from the outside of the casing to the interior of the hollow and fixed by bonding to the periphery of the opening. As a result, the microphone according to the present invention has a high mass-productivity.
Furthermore, in the microphone of the present invention, the casing may contain the solid state device in parallel to the combination of diaphragm and fixed electrode in its hollow. The combination and the solid state device are arranged laterally so that the whole thickness of the casing is further reduced to make the microphone much thinner.
In the present invention, preferably, the hollow portion of the casing may be divided into two partial hollow portions, in which case the combination of the fixed electrode and the diaphragm may be located in one of the hollow portions and the solid state device is placed in the other hollow portion.
Furthermore, it is preferable that the microphone according to the present invention should have cover for covering the opening of the hollow portion. The cover can also be shared by the fixed electrode, where the cover and the diaphragm are arranged facing to each other to constitute a capacitance gap.
In the present invention, one or more recesses for absorbing a sound preferably may be formed on the bottom of the hollow portion of casing, to eliminate an influence on the diaphragm of a reflected wave to be generated with deduced volume of the hollow portion. The recesses means a depessed portion which is at a lower level than a bottom level of the hollow portion accommodating the solid state device. The vibration of the diaphragm caused by the sound wave makes a wave to the back side toward the hollow bottom. The recess serves to increase a volume under the diaphragm the in the casing to form a release passage for the reflected wave which passage prevents the reflected wave of the diaphragm from interference with the diaphragm, and to compensate for a deterioration in sound-vibration fidelity of the diaphragm.
In the present invention, moreover, the casing can directly apply leads, or interconnections on an outer surface and/or inner surface thereof by utilizing dielectric properties. The dielectric casing may be provided with pad electrode on the outside of the case to be connected to these leads preferably, the pad electrodes can be applied on both sides of the bottom surface of the casing with a simple structure, and then the case can be mounted on other printed circuit boards by soldering the pad electrodes to the corresponding pad electrodes arranged on the circuit board.
In the present invention, furthermore, one or two stepped portions preferably may be provided on the inner wall surface of the hollow portion of the casing to set a diaphragm and a fixed electrode wherein their peripheral edge portions are mounted on the stepped portions. These parts can be conveniently fixed only by bonding such as adhesion or solder bonding, without using other special means for fixing, in the hollow portion, resulting in reduction of the number of the parts.
In the present invention, moreover, it is preferable that the casing, particularly, the hollow portion should have a rectangular cross section. The square shape can be coincident with the external shape of the solid state device or integrated circuit to be accommodated.
Furthermore, the square casing can cause the microphone to be easily handled in a manufacturing process and the completed microphone to be positioned precisely on a wiring board.